Don't worry, it's probably an easy fix. Whenever you mess with the hydraulic side of the braking or clutch system, you risk letting in air. What happens? Lots of lever travel, but no braking effort.
So why does it occur, and how do I solve it?
The first thing to remember is you can't push a gas (you can compress it, but that's a whole different subject). If air is introduced into the system, when you pull the brake lever, all you are effectively doing is momentarily compressing a gas. You aren't forcing a liquid (brake fluid) down the hoses to the brake calipers.
Master cylinders, and what's in them.
It all starts at the master cylinder. This consists of a piston and two rubber cups which seal against the bore of the cylinder.
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| GSX750ESD Rear Master Cylinder Rebuild |
The outermost cup (seal) stops fluid flowing out of the back of the master cylinder, and the innermost cup pushes the fluid out to your brakes. When you pull the lever, or push down with your foot, the piston moves through the master cylinder's bore, forcing fluid out through to the hoses and into the brake caliper.
The cylinder contains two drillings - an inlet port and a smaller bypass port. Filled via the reservoir above, the inlet port is always open and flows fluid into the piston area (between the two cups). The bypass port, just in front of the innermost cup, allows fluid to return to the reservoir when the brake is released. This port is closed (or diverted) as soon as the piston moves through its stroke, creating a sealed chamber in which to push the fluid out. Perfect.
But, if you have air in the cylinder and the system is sealed up, how can you get it out?
Bleed it? Got it in one.
Sounds easy, and most of the time it is. Unless the air actually prevents the fluid from dropping into the cylinder in the first place. This is the most common problem when starting from scratch. You see, even a slightly worn master cylinder will pump if the fluid is in front of the rubber cups. But if a port is blocked with debris, or you're pushing and compressing fresh air, the fluid stays up in the reservoir and the swearing starts.
"Bleeding brakes!" and so on...
Where to start.
Firstly, cover any part of your bike around the hydraulic system that is painted - brake fluid will work like paint stripper. It's horrible stuff.
Remove the bleed screw from the caliper and make sure it isn't blocked.
Now wrap PTFE tape around the threads of the screw (wrap it in a way that it doesn't bunch or unwrap when you screw it back in). This serves two purposes:
1) It prevents corrosion between the threads of the caliper and screw.
2) It stops air travelling past the threads when bleeding.
Get a jam jar or similar and a piece of plastic, or rubber, tube that's a tight fit on the bleed screw. It's good to have some brake fluid at the bottom of the jar to cover the end of the tube (this will stop any air going back into the system).
Fill the reservoir with new fluid out of a sealed container. Brake fluid is hygroscopic and will soak up moisture from the air like a sponge. It needs to be good quality, your life's going to depend on it. Leave the top off the reservoir so you can refill easily.
Get a ring spanner on the bleed screw, slacken it and then nip it up. You only need to open it half a turn to allow the air out so position the spanner so you can go from locked-up to half-open easily. When you're ready to pump, open the screw and push, or pull, the brake lever right through its travel. Then hold it there!
Tighten the screw, and then allow the lever to return slowly back to the starting position. Hopefully you will see the fluid level drop in the reservoir while you release the lever. This is because fluid is being drawn into the cylinder by the returning piston. Repeat the process until no further bubbles appear out of your bleed tube (still under the fluid in your jam jar).
Once satisfied, making sure the bleed screw is tight, operate the lever several times to push the pistons in the caliper right out until they're stopped by brake pad against disc. You should have a solid lever. Success.
Can't get any fluid at the caliper?
If after several pumps, there's still no fluid being pumped, you'll need to start at the master cylinder. Slacken the banjo bolt which attaches the brake hose to the cylinder - half a turn is plenty. (Make sure you have plenty of rag around it.)
Pull, or push, the brake lever through its full travel and lock up the banjo bolt. Release the lever and slacken the banjo bolt again. Repeat this process until fluid is pumped out through the union. Once you're at this stage, go back to the caliper and bleed from there.
The main thing with bleeding brakes is to lock up the bleed screw or bolts before releasing the lever - this process literally sucks fluid down from the reservoir. If you don't lock up the screw first, air is dragged back into the cylinder!
Tips when all else has failed!
1) Try removing the reservoir cap then lightly flicking the lever, letting it fly back to its stop under its own steam. You should see air bubbles rising out of the cylinder. Air pockets often form around the brake hose union, especially when they're placed too high by the manufacturer.
2) Still struggling to pump any fluid? A syringe on the end of your bleed tube should suck the necessary fluid from reservoir, via master cylinder through to caliper.
3) If all else fails, lifting the calipers above the level of the master cylinder can help shift the air bubbles, but is a little more awkward. (I've never had to resort to this technique yet.)
If none of the above can get your brakes working, you need to consider the following:
1) Is the master cylinder ok? If there's any fluid leakage evident from the lever side, it needs rebuilding or replacing.
2) Before you stripped it down, was the brake fluid in it clean? Or was it black and degraded? If it's black, the seals are starting to break down and discolour the fluid. It might still be ok with fresh fluid, but will need a complete flush. If it's impossible to get fluid to pump, rebuild the master cylinder or replace.
Seal kits and fitting.
A seal kit is quite easy to fit. Usually a circlip pliers and a small screwdriver will sort it out, but you'll almost certainly need to hone the bore to provide a smooth, even surface for the new cups.
For this you can make yourself a simple, miniature hone. Just cut a slot in a piece of 6mm round bar and fit a strip of 320 wet&dry paper in the end. Spray it with WD40 and spin it up and down the bore with a drill. And repeat. Change the paper, keep it clean with loads of WD40 and check the bore. Major grooves in the bore mean it's time for a new cylinder. Check these pics out of an old GS425 cylinder where I needed to make my own hone:
Cut a slot in a 6mm piece of round bar or similar.
Tear as strip of wet & dry (this is 320 grit) and place it in the slot.
Wrap it round in the direction of your drill (so that it doesn't unravel).
Use plenty of WD40 to provide lubrication, prevent clogging and allow any particles to flow away from the bore.
Take it steady, and keep checking the bore. As soon as you can see there are no major wear marks you should be good to go. Flush the cylinder completely with brake cleaner and make sure there is no grit, oil etc. left behind.
Now if you still don't have brakes, get in touch. Leave a comment and we'll work out just what it is your bike needs. More soon...
One last thing, don't go mad when tightening bleed screws or banjo bolts. If in doubt, ask someone in the know. Your bleed screws might only have a 7x1mm thread and they won't take abuse lightly!
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